(1) Field of the Invention
The present invention relates to a method of non-destructive inspection for resinous automotive bumper beams.
(2) Description of the Related Art
Recently, resinous automotive bumper beams, which have been obtained by heating and softening a plurality of elongate stampable sheets formed of a long-fiber-reinforced composite thermoplastic material containing, for example, longitudinally-stretching fibers as a reinforcement to laminate them and then press-molding the resulting laminate, have begun to be used in place of conventional metallic automotive bumper beams with a view toward reducing their weight and improving their capability of absorbing shock.
The bumper beams for automobiles are first of all required to have a function capable of reducing shock in a collision or the like and to absorb striking energy to at least a predetermined standard value upon a collision. In this case, since the bumper beams absorb the striking energy by their bending, they are secondarily required not to bend to such an extent that they strike on the automotive bodies before they absorb the energy to the predetermined standard value, namely, to have flexural rigidity capable of absorbing the energy to the predetermined standard value in the bent condition below a predetermined stroke depending upon a distance in design between the bumper beams and the automobiles, etc.
For this reason, bumper beams have heretofore been inspected during production as to whether they meet the above-described two requirements or not, i.e., with respect to their striking energy-absorbing capability and flexural rigidity, in the following manner. Namely, a plurality of bumper beams are optionally taken out of the bumper beams produced as samples for inspection. Each of the bumper beams thus extracted is subjected to the so-called destructive test, as illustrated in FIG. 6, by fixedly supporting both ends of the bumper beam B on a pair of receiver jigs a in the same state as installed on an automobile (not shown) and then applying a flexural load by a pressing jig c having a pressing surface b of a predetermined radium of curvature such as a pole to the center of the bumper beam B until it is destroyed, thereby measuring the flexural load on and the bend stroke (hereinafter referred to as "bending strain") of the bumper beam B at the time it is destroyed.
More specifically, in the measurements of the flexural load on and the bending strain of the bumper beam B by the destructive test, as illustrated in FIG. 7 by way of example, the measured value as to the bending strain S of the bumper beam B linearly increases until just before the bumper beam B is destroyed as the flexural load F increases, and upon the destruction of the bumper beam B, it sharply increases in association with the deterioration in flexural rigidity of the bumper beam B, resulting in the destruction of the bumper beam B. In this case, supposing the absorption of energy in the bumper beam B is E, the following relationship is generally satisfied: EQU E=(1/2).multidot.F.multidot.S (1)
Therefore, the first requirement can be represented by the following relational expression: EQU Ed.gtoreq.Ea (2)
wherein Ea means a control value for determining whether the energy absorption in the destructive test is satisfactory or not, and Ed denotes an energy absorption at the moment the bumper beam B is destroyed. Here, the control value Ea as to the energy absorption corresponds to the predetermined standard value of the striking energy and is determined by various experiments according to the receiver jigs a and the pressing jig c.
Therefore, in order for the bumper beam B to meet the first requirement, a measuring point Pd of the flexural load Fd and bending strain Sd upon the destruction must lie in the upper side (on the drawing) of a hyperbola [hereinafter referred to as "hyperbola (3)"] represented by the following equation: EQU (1/2).multidot.F.multidot.S=Ea (3)
as indicated, for example, by a solid line in FIG. 7. On the basis of this required condition, the bumper beam B is inspected as to whether it meets the first requirement or not.
On the other hand, the second requirement means that supposing a control value as to the bending strain, which corresponds to the predetermined stroke in the destructive test and serves to determine whether it is satisfactory or not, is Sa, the energy absorption E is calculated in accordance with the equation (1) in a state where the bending strain S of the bumper beam B in the destructive test is not greater than the control value Sa of the bending strain and is not smaller than the control value Ea of the energy absorption. Here, the control value Sa of the bending strain is also determined by various experiments according to the receiver jigs a and the pressing jig c in the same manner as in the control value Ea of the energy absorption.
Therefore, in order for the bumper beam B to meet the second requirement, supposing the flexural load at the control value Sa as to the bending strain on the hyperbola (3) is Fa as shown in FIG. 7, a measured value Sx of the bending strain S at the flexural load Fa must be not greater than the control value Sa of the bending strain, for example, as indicated by a solid line in FIG. 7, i.e., EQU Sx.gtoreq.Sa (4)
On the basis of this required condition, the bumper beam B is inspected as to whether it meets the second requirement or not. For example, a bumper beam B in which measured values as indicated by a broken line in FIG. 7 are obtained is judged to be poor in quality because the measured value of the bending strain S under the flexural load Fa exceeds the control value Sa of the bending strain.
As described above, the striking energy-absorbing capability and flexural rigidity of the bumper beam have heretofore been inspected by performing the destructive test, in which the flexural load is applied by the pressing jig c to the bumper beam fixedly supported on the receiver jigs a, to measure the flexural load Fd on and the bending strain Sd of the bumper beam upon destruction thereof as well as the bending strain Sx under the flexural load Fa, and then comparing the energy absorption Ed upon destruction, which is calculated from the flexural load Fd and the bending strain Sd, and the bending strain Sx with the predetermined control value Ea of the energy absorption and the predetermined control value Sa of the bending strain, respectively.
However, since the inspection of the bumper beams is conducted by the destructive test, the bumper beams tested cannot be used as products. Accordingly, many bumper beams are necessarily consumed at every inspection. Regarding the resinous bumper beams in particular, this has been an obstacle to the reduction in production costs of automobiles because they are expensive in general.
Moreover, since such a destructive test has been conducted for the bumper beams taken out as samples, there has been a potential problem that even when only one of the samples is judged to be poor in quality, many bumper beams produced before the test must be scrapped. In the resinous bumper beams in particular, the thermoplastic material requires a fixed period of time from its molding into a bumper beam to its complete crystallization. Therefore, the resinous bumper beams take a longer time before they come to be subjected to the destructive test as compared with the metallic bumper beams. Accordingly, bumper beams produced prior to the test are increased in number, leading to a possibility that a great number of bumper beams must be scrapped.
In order to solve such a disadvantage, it is desired to provide an inspection method capable of testing bumper beams produced and using bumper beams, which have been judged to be good in quality after the test, as products after the test.
In this case, since a bumper beam is generally designed and produced in such a manner that the energy absorption Ed at the moment it is destroyed exceeds sharply the control value Ea of the energy absorption as illustrated in FIG. 7, and at the time the bumper beam absorbs energy to the control value Ea, the bending strain S of the bumper beam falls within the range in which it linearly changes relative to the flexural load F and the bumper beam exhibits restoring property upon unloading it, it is considered, for example, that the above-described test is stopped at the time when the bumper beam absorbs energy to the control value Ea without continuing the test until the bumper beam is destroyed.
However, in the above inspection method, the bumper beam B is fixedly supported at both ends thereof on the receiver jigs a in the same state as installed on an automobile as illustrated in FIG. 6, and the flexural load is applied by pressing the pressing jig c having the pressing surface b of the predetermined radium of curvature against the center of the bumper beam B. Accordingly, the center and both ends of the bumper beam B are deformed due to pressing, so that it is difficult to use the bumper beam B after the inspection as a product.
Therefore, it is also considered that the receiver jigs and the pressing jig are exchanged so as not to impair the bumper beam upon applying the flexural load. However, the exchange of the receiver jigs and the pressing jig will make the condition of the flexural load to be applied to each portion of the bumper beam differed from that in the above destructive test in general. It is hence necessary to set up again the control values of the energy absorption and flexural strain according to new receiver jigs and pressing jig. In this case, it is desired that these control values can be set up with ease.